My primary specialty first and foremost is network and server security and I personally
have more than 32 years professional experience in this field with a loyal base of
clients all over the world who usually want me specifically for all security matters
including a number who are also regular visitors here as well.

At the very least, I would be glad to take a look and tell you where
your weaknesses are and can make a few recommendations.

I can also probably tell you exactly how your server was hacked and
close that security hole for you so that it doesn't happen to you again.

On the other extreme, I can configure your server for you increasing the security
to the point where it is as close to impossible to hack as you will ever hope to get.

And last but not least, www.configserver.com are also very good. You should definitely install their mailscanner package, and their firewall is the best available at the moment (more recent than APF and regularly updated).

There are quite a few other really good companies, these are just those I've heard of often.